1. Field of the Invention
The present invention relates generally to an attitude control system for a vehicle, and, more particularly to a deadband mass expulsion attitude control system.
2. Brief Description of Related Developments
In the employment of spacecraft in a mission, such as the encirclement of the earth by a communications or scientific satellite, it is necessary to stabilize the attitude of the spacecraft. An adaptive mass expulsion attitude control system is described in U.S. patent application Ser. No.: 09/363,680, commonly assigned to the Assignee of the present invention and which is incorporated herein by reference. The stabilization is generally accomplished by overcoming the destabilizing torques of sources of disturbance to the stabilization of the spacecraft. Sources of destabilizing torque can include aerodynamic torques experienced by spacecraft travel in a low orbital path through residual atmosphere, solar torque caused by pressure from the sun, and torque induced by gravity gradient from the earth""s gravitational field.
The attitude control system may employ thrusters that, upon activation, develop forces and moments that push the spacecraft back to the desired attitude. A thruster may be constructed to expel mass, such as ionized particles accelerated by an electrostatic field, or molecules of gas expelled from a canister of pressurized gas. In addition, a control system employed for attitude stabilization may employ magnetic forces, as by use of magnetic torquers. The magnetic torquers comprise rods of magnetic material encircled by coils excited with electric current provided by photocells onboard the spacecraft, wherein the magnetic forces of the coils interact with a relatively weak magnetic field of the earth. The interaction of these magnetic forces develops a torque that tends to aid in the attitude stabilization.
A thruster attitude control system operates by expelling gas in the form of pulses of the gas. In such a thruster, the compressed gas is contained in a canister. The canister of compressed gas communicates via a solenoid-operated valve to an exterior nozzle through which the expelled molecules of gas are directed into the environment outside the spacecraft. Use of the solenoid to open the valve during the time interval of the resulting jet, followed by a closing of the valve to terminate the jet of compressed gas, provides the desired impulse of the expelled gas.
Generally, in a control system, the pulses of the expelled gas have a predetermined duration. The repetition frequency of the pulses is sufficiently low such that information obtained from attitude sensors onboard a spacecraft can be employed to monitor and predict the progress in correction of the spacecraft attitude. Thereby, the pulses of expelled gas can be generated as needed for correction and/or stabilization of the spacecraft attitude. In a xe2x80x9cdeadbandxe2x80x9d mass expulsion attitude control system, gas pulses can be generated when attitude exceeds a deadband value. These types of systems generate control torques in the form of gas thrusters that pulse in metered bursts. The thrusters generally operate at full-on or full-off. The duration of the pulse can be controlled and maintains a minimum realizable size for small signals. The pulses can be formed in a modulator logic device that can determine when an input attitude error signal exceeds a pre-set value or range, which is referred to herein as the xe2x80x9cdeadband.xe2x80x9d The xe2x80x9cdeadbandxe2x80x9d generally describes an acceptable range for variances in the attitude of the vehicle. During steady-state operation, the system ideally pulses just often enough on one side to balance over time any external torque disturbance on the vehicle or spacecraft and keep the spacecraft attitude within the limits of the deadband. A difficulty ensues when there is noise on the attitude input error signal that is significant when compared to the range of the deadband, and particularly when the attitude input error signal is approaching a limit of the deadband range. The presence of noise on the attitude input error signal can cause the execution of multiple control pulses when the attitude input error signal is approaching or near the dead-band limit, since the signal noise can repeatedly exceed the deadband limit, which in turn causes the execution of a control pulse. The multiple control pulses induce extra control torques that generate acceleration that overdrives the attitude of the vehicle. After such accelerations, the attitude control of the vehicle can proceed to the opposite side of the deadband range where further control pulses reverse the motion. The resulting multiple control pulses over-accelerate the spacecraft into motion that causes excessive limit cycle mass usage. The noise stimulated control pulses cause non-efficient and more frequent expenditure of propellant.
The present invention is directed to, in a first aspect, an attitude control system. In one embodiment, the system comprises a controller and a noise screen device coupled to the controller. The controller is adapted to control an attitude of a vehicle carrying an actuator system that is adapted to pulse in metered bursts in order to generate a control torque to control the attitude of the vehicle in response to a control pulse. The noise screen device is adapted to generate a noise screen signal in response to the control pulse that is generated when an attitude error input signal exceeds a predetermined deadband attitude level. The noise screen signal comprises a decaying offset signal that when combined with the attitude error input signal results in a net attitude error input signal away from the predetermined deadband level to reduce further control pulse generation.
In one aspect, the present invention is directed to a method of reducing an undesirable response to signal noise for a mass expulsion spacecraft control system. In one embodiment, the method comprises generating a noise screen signal in response to an initial attitude control pulse. The noise screen signal comprises a decaying offset signal that when combined with the attitude error input signal results in a net attitude error input signal away from the predetermined deadband level to reduce further control pulse generation. The noise screen signal is combined with an input attitude control signal and a subsequent attitude control pulse based on the combined noise screen signal and input attitude control signal is generated. Each subsequent control pulse is adapted to generate additional superposed decaying functions.